Electrode having a CoS layer thereon, process or preparation and uses thereof

ABSTRACT

The present invention relates to an electrode comprising a non-conductive substrate, a first layer and a second layer. The first layer is disposed on the substrate and comprises indium tin oxide or fluorine-doped SnO 2 . The second layer is disposed on the first layer and comprises CoS. A process for preparing this electrode is also disclosed. Such an electrode is particularly useful in a photovoltaic cell.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority on U.S. provisional applicationNo. 60/513,211 filed on Oct. 23, 2003, and on U.S. provisionalapplication No. 60/570,074 filed on May 12, 2004. The above-mentionedapplications are incorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to improvements in the field ofelectrochemistry. In particular, this invention relates to a CoS coatedelectrode and a process for preparing the same.

BACKGROUND OF THE INVENTION

Electrochemical photovoltaic cells (EPC's) are based on a junctionbetween a semiconductor (p-type or n-type) and an electrolyte containingone redox couple; an auxiliary electrode completes the device. If thesemiconductor and electrolyte Fermi levels are different and wellsuited, a built-in potential will develop at their interface and thedevice will exhibit diode rectification in the dark. When electrons andholes are photogenerated in the vicinity of the junction, the built-inpotential permits separation of the charges. If a n-type material isused, holes (valence band) will migrate to the interface and allowoxidation of reduced species contained in the electrolyte. At the sametime, photogenerated electrons (conduction band) will migrate toward thebulk of the semiconductor to reach the auxiliary electrode, via anexternal circuit, where they will reduce the oxidized species of theelectrolyte. If a p-type material is used, the processes are reversed:photoelectrochemical reduction at the semiconductor/electrolyteinterface, and electrochemical oxidation at the auxiliaryelectrode/electrolyte interface. As the reactions involve the same redoxcouple, there is no net chemical change in the electrolyte (ΔG=0) andtherefore the effect of the device illumination is to produce aphotocurrent and a photovoltage (photovoltaic effect). Such devices canserve as photodiodes (monochromatic light) and as solar cells (whitelight). The maximum open-circuit photopotential (V_(oc)) is determinedby the difference between the Fermi level of the semiconductor and thatof the electrolyte, the latter being fixed by the redox potential.

EPC's are very attractive for the production of electricity and presenta number of advantages over the p-n heterojunctions. The lattergenerally need a doping step and interdiffusion of majoritary carriersbetween the p and n regions, whereas the semiconductor/electrolytejunction is simply formed by transfer of majoritary carriers from thesemiconductor to the electrolyte, on immersion of the semiconductor intothe electrolyte. Among other advantages we can stress: (i) theelimination of light energy losses by absorption in one half of thejunction if the electrolyte is colorless; (ii) the possibility of usinga thin film polycrystalline semiconductor (much less expensive than asingle crystal) with only a small decrease in the cell energy conversionefficiency; (iii) the large number of redox couples (and thus ofelectrolyte Fermi levels) that can be used to vary the junction built-inpotential and hence the device photopotential and photocurrent.

There is extensive prior art on EPC's. The direct conversion of solarenergy to electricity by using a semiconductor/electrolyte interface hasbeen demonstrated by H. Gerischer and J. Goberecht in Ber. BunsengesPhys. Chem., 80, 327 (1976), and by Ellis et al. in J. Am. Chem. Soc.,98, 1635 (1976). The Gerischer cell consisted of a n-CdSe single crystalphotoanode and of a doped SnO₂ conducting glass cathode dipped in anaqueous alkaline electrolyte containing the Fe(CN)₆ ⁴⁻/Fe(CN)₆ ³⁻ redoxcouple. The energy conversion efficiency was 5% but the cell performancedecreased rapidly due to decomposition of the illuminated semiconductorelectrode. Since that time a major effort has been devoted to thetechnology of solar energy conversion and to fabrication of varioussingle crystal and polycrystalline semiconductors such as CdS, CdSe,CdTe, WS₂, WSe₂, MoS₂, MoSe₂, GaAs, CuInS₂, CuInSe₂ andCuIn_(1-x)Ga_(x)Se₂. Most of the cells used an aqueous electrolyte(various redox couples were studied: Fe(CN)₆ ⁴⁻/Fe(CN)₆ ³⁻, I⁻/I₃ ⁻,Fe²⁺/Fe³⁺, S²⁻/Sn²⁻, Se²⁻/Se_(n) ²⁻, V²⁺/V³⁺) and systems exhibiting agood energy conversion efficiency were generally unstable undersustained illumination due to a process called photocorrosion. The useof a solvent-free polymer electrolyte could eliminate the photocorrosionprocess owing to its larger electrochemical stability window and to thelow solvation energy for the ions that compose the semiconductormaterials. Furthermore, this medium allows the fabrication of compactdevices with no leakage of solvent, giving a lower absorption of visiblelight by the electrolyte. Few EPC's based on the junction betweenprotected n-Si single crystal and poly(ethylene oxide), PEO, complexedwith a mixture of KI and I₂, were investigated but their stability hasnot been demonstrated (T. A. Skotheim, Appl. Phys. Lett., 38, 712(1981), T. A. Skotheim et al. Journal de Physique, C3, 615 (1983), T. A.Skotheim and O. Inganäs. J. Electrochem. Soc., 132, 2116 (1985)).

A. K. Vijh and B. Marsan in Bull. Electrochem., 5, 456 (1989) havedemonstrated that the all-solid-state EPC's n-CdSe(polycrystalline)∥high molecular weight PEO-based copolymer (noted asmodified PEO) complexed with M₂S/xS (M=Li, Na, K; x=1, 3, 5, 7)∥indiumtin oxide conducting glass (ITO) are very stable under white lightillumination. However, these authors showed that the high seriesresistance of the cells, mainly attributed to the very low ionicconductivity of the polymer electrolytes, control the deviceperformance.

In order to enhance the conductivity of the solid electrolyte, a cesiumthiolate (CsT)/disulfide (T₂) redox couple, where T⁻ stands for5-mercapto-1-methyltetrazolate ion and T₂ for the correspondingdisulfide, was dissolved in modified PEO and studied in an EPC (J. -M.Philias and B. Marsan, Electrochim. Acta, 44, 2915 (1999)). It was foundthat the PEO¹²⁻CsT/0.1 T₂ electrolyte composition, which is transparentto visible light, exhibits the highest ionic conductivity with 2.5×10⁻⁵S cm⁻¹ at 25° C. (J. -M. Philias and B. Marsan, Electrochim. Acta, 44,2351 (1999)). Under white light illumination, the cell possesses anenergy conversion efficiency (0.11% at 50° C.) about 5 times higher thanthat of the previous configuration. The lower cell series resistance andthe more anodic potential of the T⁻/T₂ redox couple (0.52 V vs NHE ascompared to —0.34 V for the S_(n) ²⁻/S_(n+1) ²⁻ couple) are largelyresponsible for this improvement. When the EPC is illuminated, thiolateions (T⁻) are photooxidized at the n-type semiconductor electrode(forming the S—S bond of the T₂ disulfide species) whereas T₂ speciesare reduced at the conducting glass electrode (with cleavage of the S—Sbond). Despite this improvement, the conductivity of the solid polymerelectrolyte is still too low, particularly at room temperature, andcontinues to limit the cell performance. EPC's incorporating a muchhigher conductive gel electrolyte (˜10⁻³ S cm⁻¹ at 25° C.) were reportedin the literature, for example by Cao et al. in J. Phys. Chem., 99,17071 (1995), and Mao et al. in J. Electrochem. Soc., 145, 121 (1998).This type of electrolyte consists in the introduction of an aproticliquid electrolyte in a polymeric matrix. The polymer gives goodmechanical properties whereas the liquid electrolyte is responsible forthe good conductivity and electrode wetting. Renard et al. inElectrochim. Acta, 48/7, 831 (2003) found that the dissolution of theT⁻/T₂ redox couple in a mixture of DMF and DMSO, and incorporated inpoly(vinylidene fluoride), PVdF, gives transparent and highly conductivegel electrolytes (conductivities up to 7×10⁻³ S cm⁻¹ at 25° C.) withvery good mechanical properties. However, when this electrolyte replacedthe solid ionic membrane PEO¹²⁻CsT/0.1 T₂ in an EPC, the cell conversionefficiency was not improved.

It has been demonstrated that the cell performance is actually limitedby the very slow reduction kinetics of the oxidized species (T₂) at thetransparent ITO auxiliary electrode and that the difference betweenoxidation potential of T⁻ and reduction potential of T₂ at thiselectrode is as large as 3.06 V in a PVdF-based gel electrolytecontaining 50 mM CsT and 5 mM T₂. Other authors previously reported lowcathodic charge transfer between ITO and aqueous polysulfide (S²⁻,S,OH⁻) (Tenne et al., Ber. Bunsenges Phys. Chem., 92, 42 (1988)) orpolyiodide (I⁻, I₂) solutions (Tenne et al., J. Electroanal. Chem., 269,389 (1989)).

Hodes et al. in J. Electrochem. Soc., 127, 544 (1980) found that thetransition metallic sulfides Cu₂S and CoS_(x) act as goodelectrocatalysts for the polysulfide redox reactions. However, theformer is mechanically instable in the electrolyte.

U.S. Pat. No. 4,421,835 describes that cobalt sulphide can be depositedon a conducting substrate such as brass. Such a deposition is carriedout by first depositing hydrous cobalt hydroxide and then by convertingthe latter into cobalt sulphide by treating it with a sulphide solution.However, this document does not teach nor suggest how to deposit cobaltsulphide on a non-conducting substrate.

U.S. Pat. No. 4,828,942 describes a thin cobalt sulphide electrode whichcan be produced by electrodeposition of cobalt onto a brass foilfollowed by alternating anodic and cathodic treatment in polysulfidesolution. However, this document does not teach nor suggest how todeposit cobalt sulphide on a non-conducting substrate.

U.S. Pat. No. 5,648,183 describes an electrocatalytic electrodecomprising a porous material such as cobalt sulphide deposited on aporous nickel or porous brass. However, this document does not teach orsuggest how to deposit cobalt sulphide on a non-conducting substrate.

It has been shown that deposited Co(II) species can serve as anelectrocatalyst for the reduction of S_(n) ²⁻ ions on ITO electrode(Tenne et al., Ber. Bunsenges Phys. Chem., 92, 42 (1988)) or p-InPphotoelectrode (Liu et al., J. Electrochem. Soc., 129, 1387 (1982)).

A method of depositing cobalt sulfide on ITO has been reported by Tenneet al. in Ber. Bunsenges Phys. Chem., 92, 42 (1988). The latter methodconsists in immersing the substrate for a few minutes in CoCl₂ solution(≧0.1 M), rinsing in water and then immersing in a separate polysulfidesolution for a few minutes; this process can be repeated several times.However, this technique does not allow an adequate control of the COSfilm thickness.

Hodes et al. in J. Electrochem. Soc., 127, 544 (1980) reported thepreparation of a CoS thin film on stainless steel. The two-steps methodinvolves the electrodeposition, at 25° C. and for few minutes, ofCo(OH)₂ onto the metallic substrate, from an aqueous solution of CoSO₄with a potassium biphthalate buffer, at a current density that dependson the pH of the electrolyte. When immersed in a polysulfide solution,Co(OH)₂ is converted to cobalt sulfide, mainly CoS. However, when theabove method is used to form a cobalt sulfide layer on a transparentconducting glass electrode (ITO), metallic cobalt is plated on thesubstrate (instead of Co(OH)₂) during the first step, that cannot beconverted to COS by a subsequent immersion in the polysulfide solution.

Thus, it seems to be very difficult to fabricate, on ITO, COS thin filmsof easily controllable thicknesses (and therefore transparencies).

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to overcome theabove-mentioned drawbacks and to provide an electrode having thin layerof CoS thereon and a process of making the same.

According to a first aspect of the invention, there is provided anelectrode comprising:

-   -   a non-conductive substrate;    -   a first layer disposed on the substrate, the layer comprising        indium tin oxide or fluorine-doped SnO₂; and    -   a second layer disposed on the first layer, the second layer        comprising COS.

According to a second aspect of the invention, there is provided anindium tin oxide glass electrode or an indium tin oxide polymerelectrode having thereon a layer comprising CoS.

According to a third aspect of the invention, there is provided anelectrode comprising:

-   -   a non-conductive substrate;    -   a first layer disposed on the substrate, the first layer        comprising indium tin oxide or fluorine-doped SnO₂;    -   a second layer disposed on the first layer, the second layer        comprising Co(OH)₂; and    -   a third layer disposed on the second layer, the third layer        comprising CoS.

According to a fourth aspect of the invention, there is provided anindium tin oxide glass electrode or an indium tin oxide polymerelectrode having thereon a layer comprising Co(OH)₂ and another layerdisposed on the layer of Co(OH)₂, the other layer comprising CoS.

According to a fifth aspect of the invention, there is provided aprocess for preparing an electrode, comprising the steps of:

-   -   a) providing a non-conductive substrate, the substrate having an        indium tin oxide and/or a fluorine-doped SnO₂ layer thereon;    -   b) electrodepositing a layer comprising Co(OH)₂ on the indium        tin oxide or fluorine-doped SnO₂ layer; and    -   c) converting at least a portion of the layer comprising Co(OH)₂        into a layer of CoS.

According to a sixth aspect of the invention, there is provided aprocess for preparing an electrode, comprising the steps of:

-   -   a) providing an indium tin oxide glass electrode;    -   b) electrodepositing a Co(OH)₂ layer on the indium tin oxide        glass electrode; and    -   c) converting at least a portion of the Co(OH)₂ layer into a        layer of CoS.

According to a seventh aspect of the invention, there is provided aprocess for preparing an electrode, comprising the steps of:

-   -   a) providing an indium tin oxide polymer electrode;    -   b) electrodepositing a Co(OH)₂ layer on the indium tin oxide        polymer electrode; and    -   c) converting at least a portion of the Co(OH)₂ layer into a        layer of CoS.

Applicant has found that by preparing an electrode according to thefifth, sixth, and seventh aspects of the invention, it was possible toobtain an ITO electrode or a fluorine-doped SnO₂ electrodes havingthereon a thin and substantially transparent CoS layer. Moreover, thethickness of the CoS layer was substantially controllable. In fact, ithas been observed that by using these processes, it was possible toobtain uniform and homogeneous CoS layers. Such a characteristic canalso explain the transparency of the obtained electrodes. Thus, theobtained electrodes can be very interesting for uses in photovoltaiccells in view of their properties.

The expression “source of sulfur” as used herein refers to a compound ora blend of compounds capable of converting Co(OH)₂ into CoS.

The expression “substantially transparent” as used herein, whenreferring to a layer or a substrate, refers to a layer or a substratehaving a transmittance of visible polychromatic light of at least 60%,preferably of at least 70%, more preferably of at least 80%, and evenmore preferably of at least 90%. A transmittance of at least 95% ispreferred.

DESCRIPTION OF PREFERRED EMBODIMENTS

In the electrode according to the first and third aspects of theinvention, the non-conductive substrate can be a polymer substrate or aglass substrate.

In the electrodes of the invention having a polymer substrate, thelatter can comprise a polymer selected from the group consisting ofpolycarbonate, acetate, polyethylene terephthalate, polyethylenenaphthalate and polyimide. Preferably, the polymer is selected from thegroup consisting of polyethylene terephthalate and polyimide.

In the electrode according to the second aspect of the invention, theglass electrode and/or the COS layer can be substantially transparent.The electrode can further includes a layer of Co(OH)₂ disposed betweenthe indium tin oxide glass electrode and the COS layer.

In the electrodes of the invention the layer comprising CoS can have athickness of less than about 30 μm. Preferably, the thickness is lessthan about 15 or preferably less than about 5 μm. A range of about 0.25to about 4 μm is particularly preferred and a range of about 0.50 toabout 2 μm is more preferred. The layer comprising CoS preferablyconsists of CoS.

In the electrodes of the invention having a layer comprising Co(OH)₂,this layer can have thickness of less than about 30 μm. Preferably, thethickness is less than about 15 or preferably less than about 5 μm. Arange of about 0.25 to about 4 μm is particularly preferred and a rangeof about 0.50 to about 2 μm is more preferred. The layer comprisingCo(OH)₂ preferably consists of Co(OH)₂.

In the electrodes of the invention, the non-conductive substrate ispreferably substantially transparent. Moreover, the CoS layer and/or theCo(OH)₂ layer are/is also preferably substantially transparent. Theelectrode is also preferably transparent.

The electrodes of the present invention can have a transmittance ofvisible polychromatic light of at least 35%, preferably of at least 45%,more preferably of at least 60%, and even more preferably of at least65%.

According to a preferred embodiment, the electrode comprises a Co(OH)₂layer directly disposed on an ITO glass electrode or an ITO polymerelectrode, and a CoS layer directly disposed on the Co(OH)₂ layer.

In the process according to the fifth aspect of the invention, step (b)is preferably carried out by:

-   -   i) using the substrate of step (a) as a cathode and providing a        cobalt electrode as an anode;    -   ii) inserting the cathode and the anode into a cell having        therein a solution comprising a cobalt salt and a buffer; and    -   iii) applying a galvanostatic current to the solution thereby        forming a layer of Co(OH)₂ on the substrate or cathode.

The non-conductive substrate can be a glass substrate or a polymersubstrate. The substrate of step (a) preferably has a sheet resistanceof about 8 to about 15 Ω/□. When the non-conductive substrate is apolymer substrate, the current in step (iii) preferably has a densityranging from about 10 to about 15 mA/cm². The polymer substratepreferably comprises a polymer selected from the group consisting ofpolycarbonate, acetate, polyethylene terephthalate, polyethylenenaphthalate and polyimide. Preferably, the polymer is selected from thegroup consisting of polyethylene terephthalate and polyimide. Thesubstrate of step (a) can have a sheet resistance of about 8 to about 15Ω/□. The substrate in step (a) preferably has an indium tin oxide layerthereon.

In the process according to the sixth aspect of the invention, step (b)is preferably carried out by:

-   -   i) using the electrode of step (a) as a cathode and providing a        cobalt electrode as an anode;    -   ii) inserting the cathode and the anode into a cell having        therein a solution comprising a cobalt salt and a buffer; and    -   iii) applying a galvanostatic current to the solution thereby        forming a layer of Co(OH)₂ on the substrate or cathode.

The indium tin oxide glass electrode preferably has a sheet resistanceranging from about 8 to about 15 Ω/□.

In the process according to the fifth and sixth aspects of theinvention, a reference electrode can further be used. Preferably, thereference electrode is a Ag/AgCl electrode or a saturated calomelelectrode. The solution in step (ii) preferably has a pH of about 6.0 toabout 7.5 and preferably from about 6.8 to about 7.5. The solution instep (ii) can further comprises LiCl, NaCl, KCl, or CsCl. The buffer ispreferably a NH₄Cl/NH₄OH buffer. The cobalt salt is preferably selectedfrom the group consisting of cobalt acetate, cobalt chloride, cobaltnitrate, cobalt sulphate and mixtures thereof. Cobalt sulphate ispreferred. The current in step (iii) preferably has a density rangingfrom about 15 to about 30 mA/cm². The current is preferably applied fora period of time ranging from about 1 to about 120 seconds and morepreferably from about 1 to about 30 seconds.

In step (iii) according to the process defined in the fifth aspect ofthe invention, the layer of Co(OH)₂ electrodeposited on the substrate ispreferably substantially transparent.

In step (iii) according to the process defined in the sixth aspect ofthe invention, the layer of Co(OH)₂ electrodeposited on the electrode ispreferably substantially transparent.

Step (c) in the process according to the fifth and sixth aspects of theinvention, is preferably carried out by contacting the layer of Co(OH)₂with a basic solution comprising at least one source of sulfur. Thebasic solution preferably has a pH of at least 10. The pH is preferablyof about 13.0 to about 14.0. The basic solution preferably comprises Stogether with Li₂S, Na₂S, K₂S or mixtures thereof. More preferably, thebasic solution comprises S together with Na₂S. The basic solution caninclude a base selected from the group consisting of LiOH, NaOH, NH₄OH,KOH and mixtures thereof. KOH is particularly preferred and moreparticularly when using a glass substrate. Step (c) is preferablycarried out by dipping the electrode so formed in step (b) into thebasic solution for a period of time ranging from 5 to 60 minutes. Aperiod of time ranging from 15 to 30 minutes is preferred. When apolymer substrate is used, NH₄OH or a similar weak base is particularlypreferred.

It will be understood by the person skilled in the art that all thepreferred embodiments previously described for the processes of thefifth and sixth aspects of the invention are also valuable, whenapplicable, to the process of the seventh aspect of the invention.

The electrodes according to any aspects of the invention can be used asan anode or as a cathode. The electrodes of the invention can be usedfor reducing a disulfide into a corresponding thiolate. Alternatively,they can be used for oxidizing a thiolate into a correspondingdisulfide.

The disulfide can be a disulfide of formula (I):R₁—S—S—R₂in which R₁ and R₂ are same or different and selected from the groupconsisting of C₂-C₂₀ alkenyl, C₁-C₂₀ alkyl, C₂-C₂₀ alkynyl, C₆-C₂₀aralkyl, C₆-C₁₂ aryl, C₃-C₈ cycloalkyl and C₂-C₁₂ heteroaryl comprising1 to 4 heteroatoms. Preferably, R₁ and R₂ are identical. The heteroatomscan be selected from the group consisting of N, O and S.

Preferably, the disulfide is a disulfide of formula (I):R₁—S—S—R₂in which R₁ and R₂ are same or different and selected from the groupconsisting of

wherein

-   -   R₃ and R₄ are same or different and selected from the group        consisting of a hydrogen atom, halogen atom, —NO₂, —OH,        —CF₃—COR₆, —COOH, —COOR₆, —NHR₅ C₂-C₈ alkenyl, C₁-C₆ alkoxy,        C₁-C₈ alkyl, C₂-C₈ alkynyl, C₆-C₂₀ aralkyl, C₆-C₁₂ aryl, C₃-C₈        cycloalkyl and C₂-C₁₂ heteroaryl comprising 1 to 4 heteroatoms        selected from the group consisting of N, O and S,    -   R₅ is a C₁-C₈ alkyl, C₆-C₁₂ aryl, C₃-C₈ cycloalkyl, C₂-C₁₂        heteroaryl comprising 1 to 4 heteroatoms selected from the group        consisting of N, O and S, or any suitable protecting group for a        nitrogen atom, such protecting groups are known by the person        skilled in the art and are defined in T. W. Green and P. G. M.        Wuts, Protective Groups in Organic Synthesis, 3rd edition, Wiley        Interscience, New York, 1999, which is hereby incoparated by        reference,    -   R₆ is a C₁-C₈ alkyl, or a C₃-C₈ cycloalkyl, and    -   X is N, O or S.

More preferably, the disulfide is of formula (I):R₁—S—S—R₂in which R₁ and R₂ are same and selected from the group consisting of

wherein

-   -   R₃ is a C₁-C₈ alkyl or CF₃,    -   R₅ is a C₁-C₈ alkyl or a phenyl, and    -   X is N, O or S.

A particularly preferred disulfide is a disulfide wherein R₁ and R₂ are

According to another aspect of the invention, there is provided aphotovoltaic cell comprising an electrode as defined in the presentinvention.

According to still another aspect of the invention, there is provided aphotovoltaic cell comprising an anode, an electrolyte and, as a cathode,an electrode as defined in the present invention. The anode can comprisea n-type semiconductor. The n-type semiconductor is preferably n-CdSe.The electrolyte can comprise a redox couple together with a solvent, apolymer, a gel or a combination thereof. The redox couple is preferablyR₁SM/(R₁S)₂ in which:

-   -   M is a metal selected from the group consisting of Li, Na, K and        Cs;    -   R₁S⁻ is a thiolate and (R₁S)₂ is a corresponding disulfide        wherein R₁ is selected from the group consisting        wherein    -   R₃ and R₄ are same or different and selected from the group        consisting of a hydrogen atom, halogen atom, —NO₂, —OH,        —CF₃—COR₆, —COOH, —COOR₆, —NHR₅ C₂-C₈ alkenyl, C₁-C₆ alkoxy,        C₁-C₈ alkyl, C₂-C₈ alkynyl, C₆-C₂₀ aralkyl, C₆-C₁₂ aryl, C₃-C₈        cycloalkyl and C₂-C₁₂ heteroaryl comprising 1 to 4 heteroatoms        selected from the group consisting of N, O and S,    -   R₅ is a C₁-C₈ alkyl, C₆-C₁₂ aryl, C₃-C₈ cycloalkyl, C₂-C₁₂        heteroaryl comprising 1 to 4 heteroatoms selected from the group        consisting of N, O and S, or any suitable protecting group for a        nitrogen atom,    -   R₆ is a C₁-C₈ alkyl, or a C₃-C₈ cycloalkyl, and    -   X is N, O or S.

More preferably, the redox couple is R₁SM/(R₁S)₂ in which:

-   -   M is a metal selected from the group consisting of Li, Na, K and        Cs;    -   R₁S⁻ is a corresponding disulfide wherein R₁ is selected from        the group consisting of        wherein    -   R₃ is a C₁-C₈ alkyl or CF₃    -   R₅ is a C₁-C₈ alkyl or a phenyl, and    -   X is N, O or S.

Even more preferably, R₁ is

According to another aspect of the invention, there is provided a methodfor reducing disulfides into thiolates comprising the step ofelectrochemically reducing the disulfides by means of any one of theelectrodes of the present invention.

According to another aspect of the invention, there is provided a methodfor oxidizing thiolates into disulfides, comprising the step ofelectrochemically oxidizing the thiolates by means of any one of theelectrodes of the present invention.

The electrodes of the invention can also be used for reducing atriiodide (I₃ ⁻) or iodine (I₂) into an iodide (I⁻). Alternatively, theycan be used for oxidizing an iodide (I⁻) into a triiodide (I₃ ⁻) oriodine (I₂).

The iodide (I⁻) can be provided from a compound of formula (IIA):M⁺I⁻in which M⁺ is a metal selected from the group consisting of Li⁺, Na⁺,K⁺, Rb⁺and Cs⁺.

The electrodes of the present invention can be used for catalyzingoxidation/reduction reactions for a redox couple. The redox couple canbe M⁺/I₃ ⁻ or M⁺I⁻/I₂ in which M⁺ is a metal selected from the groupconsisting of Li⁺, Na⁺, K⁺, Rb⁺ and Cs⁺. The redox couple can bedissolved in liquid organic solvents like ethylene carbonate (>37° C.),propylene carbonate, ethylmethyl carbonate, dimethyl carbonate, diethylcarbonate, methoxyacetonitrile, acetonitrile, N,N-dimethylformamide,dimethyl sulfoxide, methoxypropionitrile, 3-methyl-2-oxazolidinone andmixtures thereof. The electrolytic solution (hereafter called theelectrolyte) can also be incorporated in silica nanoparticles or apolymer to form a gel. Examples of such compounds include poly(ethyleneglycol), poly(ethylene oxide), poly(acrylonitrile),poly(epichlorohydrin-co-ethylene oxide), poly(methyl methacrylate) andpoly(vinylidenefluoride-co-hexafluoropropylene). Another possibility isto incorporate the redox couple in a solvating polymer to form a solidpolymer electrolyte. Examples of such compounds include poly(ethyleneoxide) and polyphosphazene. The concentration of compound of formula(IIA) is between about 0.05 M and 0.9 M, and iodine is at aconcentration of at least 0.005 M. More preferably, the electrolyte isKI/I₂ (50 mM/5 mM) and is dissolved in N,N-dimethylformamide anddimethyl sulfoxide (60/40).

Also, the iodide (I⁻) can be provided from a compound of formula (IIB):T⁺I⁻in which T⁺ is an organic cation and preferably an heterocyclic cation.

Alternatively, the redox couple can thus be T⁺I⁻/I₃ ⁻ or T⁺I⁻/I₂. Thecompound of formula (IIB) can be provided in the form of an ambienttemperature ionic liquid (also known as ambient temperature molten saltor room-temperature ionic liquid (RTIL)), which can consist of anheterocyclic cation based on substituted imidazole and an iodide anion.

The compound of formula (IIB) can be a compound of formula

in which R₁ and R₃ are same or different and selected from the groupconsisting of C₁-C₉ alkyl and benzyl, and R₂ is a C₁-C₉ alkyl or H.

In a particular embodiment, some of these compounds of formula (III) arenot liquid at ambient temperature so they have to be dissolved inorganic solvents or ionic liquids comprising an anion that is notiodide. Examples of such anions are halogen atoms, polyiodides (I₂ ⁻, I₃⁻, I₅ ⁻, I₇ ⁻, I₉ ⁻ and I₁₁ ⁻), PF₆ ⁻, BF₄ ⁻,bis(trifluoromethanesulfonyl)amide, trifluoromethanesulfonate,dicyanamide, AlCl₄ ⁻, ClO₄ ⁻, NO₃ ⁻, CH₃COO⁻, CF₃COO⁻, C₄F₉SO₃ ⁻, 2.3HF, 2,2,2-trifluoro-N-(trifluoromethylsulfonyl)acetamide, CH₃SO₃ ⁻,CH₃C₆H₄SO₃ ⁻, AsF₆ ⁻, CF₃SO₃ ⁻ and (CF₃SO₂)₃C⁻. Ionic liquids of thistype have many benefits: they can dissolve an enormous range ofinorganic, organic and polymeric materials at very high concentrations,are non-corrosive, have low viscosities and no significant vaporpressures.

The compound of formula (IIB) can also be a compound of formula (IV):

in which R₄ and R₅ are same or different and represent a C₁-C₆ alkyl.

Preferably, the compound of formula (IV) is 1,3-ethylmethylimidazoliumiodide (EMI-I):

The electrolyte can thus be EMI-I/I₂ (163 mM/10 mM) dissolved inEMI-TFSI (trifluoromethyl sulfonylimide).

When using a compound of formula (III) or (IV) in a solar cell, itsconcentration is comprised between 0.05 and 0.9 M, when the compound isin a solid form. For iodine, the concentration is between 5 and 100 mM.When the compound is liquid at room temperature, it is used thereof anddissolves a concentration of iodine comprised between 5 and 500 mM.

The compound of formula (IIB) can also be a compound of formula (V):

in which R₆ and R₇ are same or different and selected from the groupconsisting of a hydrogen atom and a C₁-C₇ alkyl.

The compound of formula (IIB) can also be a compound (pyrroliniumcation) of formula (VI) which lies between the fully saturatedpyrrolidinium cation and the semi-aromatic imidazolium cation:

in which R₈ and R₉ are same or different and selected from the groupconsisting of hydrogen atom and C₁-C₄ alkyl.

The compound of formula (IIB) can also be a compound of formula (VII):

in which R₁₀, R₁₁ and R₁₂ are same or different and represent a C₁-C₁₂alkyl. Preferred compounds of formula (VII) are: (Et₂MeS)I, (Bu₂MeS)Iand (Bu₂EtS)I, which are in liquid form at room temperature.

The compound of formula (IIB) can also be a compound of formula (VIII):

in which R₁₃ is selected from the group consisting of a hydrogen atom, ahalogen atom and a C₁-C₁₈ alkyl.

The compound of formula (IIB) can also be a compound of formula (IX):

in which R₁₄ and R₁₅ are same or different and represent a C₁-C₃ alkyl.

The compound of formula (IIB) can also be a compound of formula (X):

in which R₁₆ to R₁₉ are same or different and selected from the groupconsisting of hydrogen atom, C₁-C₁₂ alkyl (preferably isopropyl), C₂-C₆alkoxyalkyl (preferably methoxymethyl and ethoxymethyl), C₃ alkenyl andC₃ alkynyl.

In the photovoltaic cell of the invention comprising redox couple, thelatter can also be a redox couple comprising an iodide of formula (IIA)or (IIB) as previously defined, with I₃ ⁻ or I₂.

According to another aspect of the invention, there is provided a methodfor reducing a triiodide (I₃ ⁻) or iodine (I₂) into an iodide (I⁻)comprising the step of electrochemically reducing the triiodide oriodine by means of any one of the electrodes of the present invention.

According to another aspect of the invention, there is provided a methodfor oxidizing an iodide (I⁻) into a triiodide (I₃ ⁻) or iodine (I₂),comprising the step of electrochemically oxidizing the iodide by meansof any one of the electrodes of the present invention.

According to another aspect of the invention, there is provided a methodfor catalyzing oxidation and reduction reactions of a redox couple offormula R₁SM/(R₁S)₂, as previously defined, comprising the step ofsubmitting the redox couple to an electrical current between at leasttwo electrodes wherein at least one of the electrodes is an electrode asdefined in the present invention.

According to another aspect of the invention, there is provided a methodfor catalyzing oxidation and reduction reactions of a redox couple offormula M⁺I⁻/I₃ ⁻ or M⁺I⁻/I₂, as previously defined, comprising the stepof submitting the redox couple to an electrical current between at leasttwo electrodes wherein at least one of the electrodes is an electrode asdefined in the present invention.

According to another aspect of the invention, there is provided a methodfor catalyzing oxidation and reduction reactions of a redox couple offormula T⁺I⁻/I₃ ⁻ or T⁺I⁻/I₂, as previously defined, comprising the stepof submitting the redox couple to an electrical current between at leasttwo electrodes wherein at least one of the electrodes is an electrode asdefined in the present invention, and wherein T⁺ is as previouslydefined.

Further features and advantages of the invention will become morereadily apparent from the following description of preferred embodimentsas illustrated by way of examples in the appended drawings wherein:

FIG. 1 shows cyclic voltammograms comparing an ITO on glass electrodewith an ITO on glass/Co(OH)₂/CoS electrode according to a preferredembodiment of the invention;

FIG. 2 shows other cyclic voltammograms comparing a Pt electrode with anITO on glass/Co(OH)₂/CoS electrode according to a preferred embodimentof the invention;

FIG. 3 shows still other cyclic voltammograms comparing an ITO on glasselectrode with an ITO on glass/Co(OH)₂/CoS electrode according to apreferred embodiment of the invention;

FIG. 4 shows further cyclic voltammograms comparing a Pt electrode withan ITO on glass/Co(OH)₂/CoS electrode according to a preferredembodiment of the invention;

FIG. 5 is a plot showing the influence of Co(OH)₂ and CoS layers on anITO on glass electrode in an Electrochemical Photovoltaic Cell (EPC) indarkness;

FIG. 6 is a plot showing the influence of Co(OH)₂ and CoS layer on anITO on glass electrode in the Electrochemical Photovoltaic Cell of FIG.5, under a polychromatic light; and

FIG. 7 shows still further cyclic voltammograms demonstrating theinfluence of the deposition time of the Co(OH)₂ layer in an ITO onglass/Co(OH)₂/CoS electrode according to a preferred embodiment of theinvention;

FIG. 8 shows X-Ray Photoelectron Spectroscopy (XPS) spectra (analysis ofsulphur) carried out on an ITO on glass/Co(OH)₂ electrode and an ITO onglass/Co(OH)₂/CoS electrode according to a preferred embodiment of theinvention;

FIG. 9 shows an X-ray Diffraction (XRD) pattern of an ITO onglass/Co(OH)₂ electrode prepared by a method which constitutes apreferred embodiment of the invention;

FIG. 10 shows an XRD pattern of a chemically prepared Co(OH)₂ powder;

FIG. 11 shows a visible absorption spectrum of an ITO on glasselectrode;

FIG. 12 shows visible absorption spectra of ITO on glass/Co(OH)₂/CoSelectrodes according to preferred embodiments of the invention;

FIG. 13 shows a visible absorption spectrum of a gel electrolyte;

FIG. 14 shows cyclic voltammograms comparing an ITO on glass electrodewith an ITO on glass/Co(OH)₂/CoS electrode in a DMF/DMSO (60/40)/0.1 MTBAP solution comprising 50 mM of KI and 5 mM of I₂ according to apreferred embodiment of the invention;

FIG. 15 shows other cyclic voltammograms demonstrating the influence ofthe NaCl concentration in the electrodepositing solution utilized forthe electrodeposition of the Co(OH)₂ layer on ITO on glass, to prepareITO on glass/Co(OH)₂/COS electrodes, in a DMF/DMSO (60/40)/0.1 M TBAPsolution comprising 50 mM of KI and 5 mM of I₂ according to a preferredembodiment of the invention;

FIG. 16 shows still other cyclic voltammograms comparing a Pt electrodewith an ITO on glass/Co(OH)₂/COS electrode in a DMF/DMSO (60/40)/0.1 MTBAP solution comprising 50 mM of KI and 5 mM of I₂ according to apreferred embodiment of the invention;

FIG. 17 shows further cyclic voltammograms comparing an ITO on glasswith an ITO on glass/Co(OH)₂/COS electrode in a EMI-TFSI solutioncomprising 0.163 M of EMI-I and 10 mM of I₂ according to a preferredembodiment of the invention;

FIG. 18 shows still further cyclic voltammograms comparing a Ptelectrode with an ITO on glass/Co(OH)₂/CoS electrode in a EMI-TFSIsolution comprising 0.163 M of EMI-I and 10 mM of I₂ according to apreferred embodiment of the invention;

FIG. 19 shows other cyclic voltammograms demonstrating the influence ofthe electrodeposition time of the Co(OH)₂ layer on ITO on glass, toprepare ITO on glass/Co(OH)₂/COS electrodes, in a EMI-TFSI solutioncomprising 0.163 M of EMI-I and 10 mM of I₂ according to a preferredembodiment of the invention;

FIG. 20 shows a cyclic voltammogram of an ITO on polymer (polyethyleneterephthalate) electrode having a surface area of 0.05 cm²; and

FIG. 21 shows a cyclic voltammogram of an ITO on polymer (polyethyleneterephthalate)/Co(OH)₂/COS electrode having a surface area of 0.05 cm²according to a preferred embodiment of the invention.

The following non-limiting examples further illustrate the invention.

EXAMPLES

Indium tin oxide on glass electrodes and indium tin oxide on polymerelectrodes having thereon a layer of Co(OH)₂ and a layer of CoS havebeen prepared according to the following method.

1) Electrodeposition

Prior to electrodeposit a layer of Co(OH)₂ on ITO on glass obtained fromLIBBEY OWENS FORD (trade-mark), the latter is cleaned with soap andwater, rinsed with water and dried by means of acetone. Then, theelectrode is sonicated in dichloromethane for a period of 5 minutesprior to be air dried. Finally, the electrode is connected to a copperclip prior to the electrodeposition.

The electrodeposition is carried out in a cell having three electrodes(by means of a potentiostat) by applying a constant current(galvanostatic mode). The cell contains 25 mL of a solution comprising20 g/L of CoSO₄ and 1 to 2 M of NaCl. The solution also comprises 100 μLof a NH₄Cl/NH₄OH buffer in order to maintain the pH in a range of about6.8 to about 7.5. The buffer contains 1.6875 g of NH₄Cl and 3.575 g ofNH₄OH.

The ITO on glass electrode, which has a surface area of about 0.1 toabout 0.5 cm² exposed to the solution, is used as a cathode and a cobaltelectrode is used as an anode. The cobalt anode has a surface area ofabout 8 cm² and is located at about 3 cm from the cathode. A Ag/AgClreference electrode is also utilized.

The density of the cathodic current preferably ranges from 15 to 30mA/cm². By using such densities of current, the layer ofelectrodeposited Co(OH)₂ is of good quality, and can be thin andtransparent.

Such an electrodeposition can be performed by using, as a cathode, anelectrode comprising a polymeric material (or polymer substrate) havinga layer of ITO thereon. When such an electrode is used, it is firstcleaned with soap and water in an ultrasonic cleaner for a period of 15minutes, rinsed with water, sonicated in water for another period of 15minutes and air dried. The electrodeposition is carried out on a samplehaving a surface area of about 0.1 to about 1 cm² exposed to thesolution and using the same type of cell than that utilized for theelectrodeposition on an ITO on glass electrode. The cathodic currentpreferably ranges from 10 to 15 mA/cm². It has been noted that theCo(OH)₂ layers tend to better adhere to polymeric materials than to aglass material. Moreover, since the polymeric materials used aregenerally foldable, the electrodes of the invention which include apolymeric material substrate can be folded or rulled up, which makesthem particularly interesting for the manufacture of low-cost solarcells.

Various electrodeposition times have been investigated in order toprovide optimized electrodes. Interesting results have been obtained byelectrodepositing the layer of Co(OH)₂ over a period of time rangingfrom 1 to 90 seconds. Preferably, the period of time ranges from 1 to 30seconds. Such periods of time permit to better control the thickness ofthe layer and hence its transparency.

The thickness of the Co(OH)₂ layer has been measured by means of amicrometer and confirmed using a scanning electron microscope (SEM). Thevalues range preferably from about 0.25 to about 4 μm.

2) Conversion of at Least a Portion of the Co(OH)₂ Layer into a CoSLayer

This second step is carried out by dipping the ITO on glass/Co(OH)₂electrode obtained in step (1) into a solution comprising 1 M Na₂S, 1 MS and 1 M KOH. This solution is prepared by successively dissolving inwater KOH, Na₂S and S. The dipping of the electrode is carried out overa period of time of about 30 minutes. When a CoS layer is formed, thecolor of the surface of the electrode changes from blue-green to black.After the conversion, the electrode is rinsed with nanopure water andthen dried under vacuum for a period of about 12 hours.

Such a conversion can also be performed by dipping an electrodecomprising a polymeric material having a layer of ITO and a layer ofCo(OH)₂ thereon into a solution comprising 0.1 M Na₂S, 0.1 M S and 0.1 MNH₄OH. When such an electrode is used, the dipping is carried out overthe same period of time (30 minutes) and after the conversion, theelectrode is rinsed and dried under the same conditions than those givenabove.

When the Co(OH)₂ layer is very thin on the glass substrate or on thepolymeric substrate, it is possible to obtain a substantially completeconversion of this layer into a CoS layer.

In order to better characterize the above-mentioned electrodes, severalcyclic voltammetry experiments have been carried out. FIG. 1 representscyclic voltammograms comparing an ITO on glass electrode having an areaof 0.50 cm² with an ITO on glass/Co(OH)₂/CoS electrode according to apreferred embodiment of the invention. A non-aqueous Ag/Ag⁺ (1 M AgNO₃)electrode was used as reference electrode. The ITO on glass/Co(OH)₂/CoSelectrode has an area of 0.40 cm². The Co(OH)₂ layer waselectrodeposited on an ITO on glass electrode at a current density of 20mA/cm² during 90 seconds. The reference and tested electrodes wereimmersed in a DMF/DMSO (60/40)/0.1 M TBAP solution comprising 50 mM ofCsT and 5 mM of T₂ (redox couple), and the scanning speed was 100 mV/s.T⁻ and T₂ are represented by the following formulae:

As it can be seen from FIG. 1, an ITO on glass electrode has beencompared with an ITO on glass/Co(OH)₂/CoS electrode in order todetermine the electrocatalytic properties of the latter. The comparisonshows that the CoS in the ITO on glass/Co(OH)₂/CoS electrode acts as avery good electrocatalyst for the reduction of T₂. In particular, thereduction of T₂ is favored by 0.84 V and the oxidation of T⁻ is favoredby 1.12 V when using the CoS electrode instead of the ITO on glasselectrode. The E_(pc) and E_(pa) of the CoS electrode are respectively−0.82 V et 0.25 V vs Ag/Ag⁺. The ΔE_(p) of the latter is thus 1.07 V.

FIG. 2 represents cyclic voltammograms comparing a Pt electrode havingan area of 0.025 cm² with an ITO on glass/Co(OH)₂/CoS electrodeaccording to a preferred embodiment of the invention. In this figure,the current relative to the Pt electrode was multiplied by a factor of10 (Pt×10). A non-aqueous Ag/Ag⁺ electrode was used as referenceelectrode. The ITO on glass/Co(OH)₂/CoS electrode has an area of 0.40cm². The Co(OH)₂ layer was electrodeposited on an ITO on glass electrodeat a current density of 20 mA/cm² during 90 seconds. The reference andtested electrodes were immersed in a DMF/DMSO (60/40)/1 0.1 M TBAPsolution comprising 50 mM of CsT and 5 mM of T₂, and the scanning speedwas 100 mV/s.

As it can be seen from FIG. 2, a Pt electrode has been compared with anITO on glass/Co(OH)₂/CoS electrode in order to determine theelectrocatalytic properties of the latter. This figure shows that theoxidation potential of the two electrodes is similar, whereas the CoSelectrode is slightly more electrocatalytic (by 90 mV) for the reductionprocess.

FIG. 3 represents cyclic voltammograms comparing an ITO on glasselectrode having an area of 0.50 cm² with an ITO on glass/Co(OH)₂/CoSelectrode according to a preferred embodiment of the invention. An Agwire was used as reference electrode. The ITO on glass/Co(OH)₂/CoSelectrode has an area of 0.40 cm². The Co(OH)₂ layer waselectrodeposited on an ITO on glass electrode at a current density of 20mA/cm² during 1 second. Both electrodes were immersed in gel comprising20% of PVdF and 80% of DMF/DMSO (60/40). The gel also comprises 50 mM ofCsT and 5 mM of T₂, and the scanning speed was 100 mV/s. The gelelectrolyte was prepared as described by Renard et al. in Electrochim.Acta, 48/7, 831 (2003).

FIG. 4 represents cyclic voltammograms comparing a Pt electrode havingan area of 0.025 cm² with an ITO on glass/Co(OH)₂/CoS electrodeaccording to a preferred embodiment of the invention. In this figure,the current relative to the Pt electrode was multiplied by a factor of10 (Pt×10). An Ag wire was used as reference electrode. The ITO onglass/Co(OH)₂/CoS electrode has an area of 0.40 cm². The Co(OH)₂ layerwas electrodeposited on an ITO on glass electrode at a current densityof 20 mA/cm² during 1 second. Both electrodes were immersed in gelcomprising 20% of PVdF and 80% of DMF/DMSO (60/40). The gel alsocomprises 50 mM of CsT and 5 mM of T₂, and the scanning speed was 100mV/s.

As it can be seen from FIGS. 3 and 4, the ITO on glass/Co(OH)₂/CoSelectrode has better electrocatalytic properties in a gel than in aliquid medium. The results obtained in FIGS. 3 and 4 are summarized inTable 1. TABLE 1 E_(pa) E_(pc) ΔE_(p) Electrodes (V vs Ag) (V vs Ag) (V)ITO on glass 2.01 −1.05 3.06 Pt 1.34 −0.36 1.70 ITO on glass/Co(OH)₂/1.13 −0.23 1.36 CoS

FIG. 5 represents the current-potential curve, obtained in darkness, oftwo Electrochemical Photovoltaic Cells: n-CdSe∥PVdF(20%)/DMF/DMSODMF/DMSO (60/40)/1.34 M CsT/0.13 M T₂∥ITO on glass andn-CdSe∥PVdF(20%)/DMF/DMSO (60/40)/1.34 M CsT/0.13 M T₂∥ITO onglass/Co(OH)₂/CoS. The n-CdSe electrodes were prepared as described byPhilias and Marsan in Electrochim. Acta, 44, 2915 (1999). The gelelectrolyte was prepared as described by Renard et al. in Electrochim.Acta, 48/7, 831 (2003), but using the above redox speciesconcentrations. EPC's were prepared in a glovebox. 60 μL of the gelelectrolyte were put into the hole (2 cm²) of a 100 μm thick paraffinfilm set at the surface of an ITO on glass or an ITO onglass/Co(OH)₂/CoS electrode. The n-CdSe electrode was then put incontact with the gel and a piece of glass was put on top of theassembly. Finally, the cell was sealed using epoxy glue. The Co(OH)₂layer was electrodeposited on an ITO on glass electrode at a currentdensity of 20 mA/cm² during 1 second. The potential measured (V) is thepotential applied to the n-CdSe electrode and the scanning speed was 1mV/s.

As it can be seen from FIG. 5, the rectification ratio (|i⁻|/i₊) at 0.8V increases from 1.0 (ITO on glass electrode) to 12.0 (ITO onglass/Co(OH)₂/CoS) which confirms that the quality of the junction isimproved when using a CoS electrode.

FIG. 6 represents the current-potential curve, obtained under apolychromatic light (incident power density from a tungsten-halogenlamp: 100 mW/cm²) of two Electrochemical Photovoltaic Cells:n-CdSe∥PVdF(20%)/DMF/DMSO (60/40)/1.34 M CsT/0.13 M T₂∥ITO on glass andn-CdSe∥PVdF(20%)/DMF/DMSO (60/40)/1.34 M CsT/0.13 M T₂∥ITO onglass/Co(OH)₂/CoS. The Co(OH)₂ layer was electrodeposited on an ITO onglass electrode at a current density of 20 mA/cm² during 1 second. Thepotential measured (V) is the potential applied to the n-CdSe electrode,the scanning speed was 10 mV/s and the light area was 1.3 cm².

As it can be seen from FIG. 6, the important increase of thephotocurrent, when using the CoS electrode in the cell, means that thereduction of T₂ is improved in a significant manner. These resultsdemonstrate that the use of a CoS electrode improves the catalyticperformance of the cell.

FIG. 7 shows still further cyclic voltammograms demonstrating theinfluence of the deposition time of the Co(OH)₂ layer in an ITO onglass/Co(OH)₂/CoS electrode according to a preferred embodiment of theinvention. The Co(OH)₂ layers were electrodeposited on an ITO on glasselectrode at a current density of 20 mA/cm². The ITO onglass/Co(OH)₂/CoS electrodes have an area of 0.10 cm². An Ag wire wasused as reference electrode. All the electrodes were immersed in aDMF/DMSO (60/40)/0.1 M TBAP solution comprising 50 mM of CsT and 5 mM ofT₂, and the scanning speed was 100 mV/s.

As it can be seen from FIG. 7, in comparison with FIG. 1, the ΔE_(p) ofthe ITO on glass/Co(OH)₂/CoS electrode has been improved from 1.07 V(deposition time of 90 seconds) to 0.99 V (deposition time of 60seconds), a difference of 80 mV. It can be seen that the ΔE_(p) isinfluenced by the deposition time of Co(OH)₂. These results are shown inTable 2. TABLE 2 Deposition time of Co(OH)₂ ΔE_(p) (seconds) (V)  1 0.9430 0.95 60 0.99 90 1.07

FIG. 8 shows an XPS sulphur analysis comparison between a firstelectrode (an ITO on glass/Co(OH)₂ electrode) and a second electrode (anITO on glass/Co(OH)₂/CoS electrode). The Co(OH)₂ layers wereelectrodeposited on an ITO on glass electrode at a current density of 20mA/cm² during 30 seconds. The only peaks present in case of the firstelectrode are probably due to traces of sulphate used in the preparationof the latter. However, in the case of the second electrode, a peak at162.4 eV (S 2p_(3/2) transition) and a peak at 163.5 eV (S P_(1/2)transition) are observed and correspond to CoS. This analysis thusproves the existence of the CoS layer in the second electrode.

FIGS. 9 and 10 show, respectively, an X-ray diffraction pattern of anITO on glass/Co(OH)₂ electrode and an X-ray diffraction pattern of aCo(OH)₂ powder. The ITO on glass/Co(OH)₂ electrode was prepared byelectrodepositing a Co(OH)₂ layer on an ITO on glass electrode at 20mA/cm² during 90 seconds. The Co(OH)₂ powder was obtained by reactingtogether Co(NO₃)₂ and KOH. These analyses have been performed in orderto determine if the Co(OH)₂ layer electrodeposited on the ITO on glasselectrode had different characteristics than Co(OH)₂ obtained in powderform. The results obtained in FIGS. 9 and 10 are summarized in tables 3and 4, respectively. TABLE 3 Relative Standard 2θ d-spacing intensityStandard relative observed calculated observed d-spacing intensity (°)(Å) (%) (Å) (%) (hkl plane) 22.3 4.63 66 4.66 70 (001) 38.1 2.74 44 2.7640 (100) 44.5 2.36 100 2.38 100 (101) 60.6 1.77 42 1.78 70 (102) 68.61.59 28 1.60 50 (110) 73.1 1.50 19 1.51 40 (111) 83.1 1.35 6 1.36 40(103  85.4 1.32 6 1.33 40 (201)

TABLE 4 Relative Standard 2θ d-spacing intensity Standard relativeobserved calculated observed d-spacing intensity (°) (Å) (%) (Å) (%)(hkl plane) 22.2 4.65 73 4.66 70 (001) 38.0 2.75 72 2.76 40 (100) 44.42.37 100 2.38 100 (101) 60.5 1.78 31 1.78 70 (102) 68.5 1.59 38 1.60 50(110) 73.0 1.50 19 1.51 40 (111) 83.0 1.35 8 1.36 40 (103  85.3 1.32 81.33 40 (201)

According to FIG. 10 and table 4, it can be seen that the d-spacingsobtained concerning the Co(OH)₂ powder are similar to those reported inthe scientific literature. Also, a preferential orientation of the (100)plane was noted. It can be concluded from FIG. 10 that Co(OH)₂ has ahexagonal phase.

In FIGS. 9 and 10 the position of the peaks is identical, whichdemonstrate that the positions are the same even if Co(OH)₂ is preparedaccording to different methods. It can also be seen from the latter twofigures that the peaks of the ITO on glass/Co(OH)₂ electrode (FIG. 9)are generally more intense and narrower than the peaks of the Co(OH)₂powder (FIG. 10). This difference indicates that electrodepositedCo(OH)₂ is more cristalline than the Co(OH)₂ powder. As example, the(101) peak in FIG. 9 has an intensity of 790 Cps, whereas the (101) peakin FIG. 10 has an intensity of 510 Cps. By comparing the peaks of FIGS.9 and 10, and by using the Scherrer relation it is possible toquantitatively establish that the crystal grains in the electrodepositedCo(OH)2 are bigger.

The visible absorption spectra of an ITO on glass electrode (FIG. 11),different ITO on glass/Co(OH)₂/CoS electrodes (FIG. 12) and of aPVdF(20%)/DMF/DMSO (60/40)/1.34 M CsT/0.13 M T₂ gel electrolyte having athickness of 100 μm (FIG. 13) are analyzed in table 5, which give thepercentage of transmitted visible polychromatic light as obtained usinga radiometer. TABLE 5 Analyzed material Visible transmitted light (%)ITO on glass electrode 75.2 ± 0.3 ITO on glass/Co(OH)₂/CoS electrode67.6 Co(OH)₂/CoS layers 89.9 ± 0.5 d.p.: 1 second at 20 mA/cm² ITO onglass/Co(OH)₂/CoS electrode 65.9 Co(OH)₂/CoS layers 87.6 ± 0.8 d.p.: 1second at 30 mA/cm² ITO on glass/Co(OH)₂/CoS electrode 62.0 Co(OH)₂/CoSlayers 82.4 ± 0.6 d.p.: 2 seconds at 20 mA/cm² ITO on glass/Co(OH)₂/CoSelectrode 46.4 Co(OH)₂/CoS layers 61.7 ± 0.4 d.p.: 3 seconds at 30mA/cm² ITO on glass/Co(OH)₂/CoS electrode 29.4 Co(OH)₂/CoS layers 39.1 ±0.4 d.p.: 5 seconds at 30 mA/cm² PVdF(20%)/DMF/DMSO (60/40)/1.34 98.9 ±0.1 M CsT/0.13 M T₂ gel electrolyted.p. = deposition time of Co(OH)₂

As it can be seen from table 5, visible light is transmitted up to 68%(transmission of 90% for the Co(OH)₂/CoS layers) when using an ITO onglass/Co(OH)₂/CoS electrode, which has been prepared byelectrodepositing Co(OH)₂ over a period of 1 second. From FIG. 12 it canalso be seen that the maximum absorbance is very low, i.e. almostnonexistent. These results demonstrate that the Co(OH)₂ deposition timestrongly influences the light transmission of the prepared electrode andthat optimal results are obtained with a deposition time of 1 second at20 mA/cm². These results also demonstrate that these specific Co(OH)₂and CoS layers have a high degree of transparency.

When using an ITO on polymer/Co(OH)₂/CoS electrode, the polychromaticvisible light transmitted is substantially the same than that in thecase of an ITO on glass/Co(OH)₂/CoS electrode. It is to be noted,however, as pointed out above, that the current density preferablyranges from 15 to 30 mA/cm² and from 10 to 15 mA/cm² when using ITO onglass and ITO on polymer, respectively.

FIG. 14 represents cyclic voltammograms comparing an ITO on glasselectrode having a surface area of 0.1 cm² with an ITO onglass/Co(OH)₂/CoS electrode according to a preferred embodiment of theinvention. In this figure, the current (I) relative to the ITO onglass/Co(OH)₂/CoS electrode was divided by a factor of 3.5 (I/3.5). Asilver wire was used as a reference electrode. The ITO onglass/Co(OH)₂/CoS electrode has a surface area of 0.1 cm². The Co(OH)₂layer was electrodeposited on an ITO on glass electrode at a currentdensity of 20 mA/cm² during 90 seconds using a solution containing 1 Mof NaCl. The reference and the tested electrodes were immersed in aDMF/DMSO (60/40)/0.1 M TBAP solution comprising 50 mM of KI and 5 mM of12 (redox couple), and the scanning speed was 100 mV/s.

As it can be seen from FIG. 14, an ITO on glass electrode has beencompared with an ITO on glass/Co(OH)₂/CoS electrode in order todetermine the electrocatalytic properties of the latter. The comparisonshows that the ITO on glass/Co(OH)₂/CoS electrode acts as a very goodelectrocatalyst for the reduction of triiodide. In particular, thereduction of I₃ ⁻ is favored by 0.86 V and the oxidation of I⁻ isfavored by 0.78 V when using the CoS electrode instead of the ITO onglass electrode. The E_(pc1) and E_(pa1) of the CoS electrode arerespectively 0.25 V and 1.01 V vs Ag. The ΔE_(p1) of the latter is thus0.76 V instead of 2.40 V for ITO on glass. TABLE 6 Electrode E_(pc1) (V)E_(pa1) (V) ΔE_(p1) (V) E_(pc2) (V) E_(pa2) (V) ΔE_(p2) (V) ITO −0.611.79 2.40 — — — ITO/ 0.25 1.01 0.76 0.97 1.52 0.55 Co(OH)₂/ CoS

FIG. 15 shows cyclic voltammogramms demonstrating the influence of theNaCl concentration in the electrodepositing solution used for theelectrodeposition of the Co(OH)₂ layers on ITO on glass, to prepare ITOon glass/Co(OH)₂/CoS electrodes. In this figure, the currents (I)relative to the ITO on glass/Co(OH)₂/CoS electrodes prepared using asolution containing 1.5 and 2 M NaCl were multiplied by a factor of 1.5(I×1.5). The Co(OH)₂ layers were electrodeposited on ITO on glasselectrodes at a current density of 20 mA/cm² during 90 seconds. The ITOon glass/Co(OH)₂/CoS electrodes have a surface area of 0.1 cm². A Agwire was used as a reference electrode. All the electrodes were immersedin a DMF/DMSO (60/40)/0.1 M TBAP solution comprising 50 mM of KI and 5mM of I₂, and the scanning speed was 100 mV/s.

As it can be seen from FIG. 15, the ΔE_(p1) of the ITO onglass/Co(OH)₂/CoS electrode has been improved from 0.76 V (NaCl 1 M) to0.56 V (NaCl 2 M), a difference of 0.20 V. This difference is mainly dueto the less positive value of E_(pa1) associated to the electrodeprepared using NaCl at a higher concentration. The same downward trendis observed for ΔE_(p2), going from 0.55 V (NaCl 1 M) to 0.28 V (NaCl 2M), a difference of 0.27 V. It can be seen that both ΔE_(p) areinfluenced by the sodium chloride concentration in the electrodepositingsolution. These results are shown in Table 7. TABLE 7 Electrode E_(pc1)(V) E_(pa1) (V) ΔE_(p1) (V) E_(pc2) (V) ZE_(pa2) (V) ΔE_(p2) (V)ITO/Co(OH)₂/CoS - 0.25 1.01 0.76 0.97 1.52 0.55 1 M NaClITO/Co(OH)₂/CoS - 0.23 0.82 0.59 0.96 1.28 0.32 1.5 M NaClITO/Co(OH)₂/CoS - 0.26 0.82 0.56 0.97 1.25 0.28 2 M NaCl

FIG. 16 represents cyclic voltammograms comparing a Pt electrode havinga surface area of 0.02 cm² with an ITO on glass/Co(OH)₂/CoS electrodeaccording to a preferred embodiment of the invention. In this figure,the current (I) relative to the ITO on glass/Co(OH)₂/CoS electrodeprepared using a solution containing 2 M NaCl was multiplied by a factorof 1.5 (I×1.5), and that of the Pt electrode was multiplied by a factorof 6.5 (I×6.5). The Co(OH)₂ layer was electrodeposited on an ITO onglass electrode at a current density of 20 mA/cm² during 90 seconds. Theconcentration of NaCl in the electrodepositing solution was 2 M. The ITOon glass/Co(OH)₂/CoS electrodes have a surface area of 0.1 cm². A Agwire was used as a reference electrode. All the electrodes were immersedin a DMF/DMSO (60/40)/0.1 M TBAP solution comprising 50 mM of KI and 5mM of I₂, and the scanning speed was 100 mV/s.

As it can be seen from FIG. 16, a Pt electrode has been compared with anITO on glass/Co(OH)₂/CoS electrode in order to determine theelectrocatalytic properties of the latter. According to this figure andTable 8, the iodide oxidation potential for the two electrodes (theredox process that occurs at the most cathodic potential) is similar,whereas the CoS electrode is more electrocatalytic (by 90 mV) for thereduction of I₃ ⁻. The ΔE_(p1) of the latter is 110 mV smaller than theone for Pt. Inversely, for the most anodic redox process (A₂/C₂), thereduction potential for the two electrodes is similar, whereas the Ptelectrode is more electrocatalytic (110 mV) for the oxidation process.The ΔE_(p2) of the latter is 60 mV smaller than the one for the CoSelectrode. TABLE 8 Electrode E_(pc1) (V) E_(pa1) (V) ΔE_(p1) (V) E_(pc2)(V) E_(pa2) (V) ΔE_(p2) (V) ITO/ 0.26 0.82 0.56 0.96 1.28 0.32 Co(OH)₂/CoS Pt 0.17 0.84 0.67 0.91 1.17 0.26

FIG. 17 represents cyclic voltammograms comparing an ITO on glasselectrode having a surface area of 0.07 cm² with an ITO onglass/Co(OH)₂/CoS electrode (0.09 cm²) according to a preferredembodiment of the invention. A silver wire was used as a referenceelectrode. The Co(OH)₂ layer was electrodeposited on an ITO on glasselectrode at a current density of 20 mA/cm² during 30 seconds using asolution containing 1 M of NaCl. The reference and the tested electrodeswere immersed in a EMI-TFSI solution comprising 0.163 M of EMI-I and 10mM of I₂. The scanning speed was 100 mV/s.

As it can be seen from FIG. 17, an ITO on glass electrode has beencompared with an ITO on glass/Co(OH)₂/CoS electrode in order todetermine the electrocatalytic properties of the latter. The comparisonshows that the ITO on glass/Co(OH)₂/CoS electrode acts as a very goodelectrocatalyst for the reduction of triiodide. In particular, thereduction of I₃ ⁻ is favored by 0.74 V and the oxidation of I⁻ isfavored by 0.77 V when using the CoS electrode instead of the ITOelectrode. The E_(pc1) and E_(pa1) of the CoS electrode are respectively0.19 V and 0.40 V vs Ag (Table 9). The ΔE_(p1) of the latter is thus0.21 V instead of 1.72 V for ITO. TABLE 9 Electrode E_(pc1) (V) E_(pa1)(V) ΔE_(p1) (V) E_(pc2) (V) E_(pa2) (V) ΔE_(p2) (V) ITO on −0.55 1.171.72 — — — glass ITO/ 0.19 0.40 0.21 0.67 0.84 0.17 Co(OH)₂/ CoS

FIG. 18 shows other cyclic voltammograms comparing a Pt electrode havinga surface area of 0.025 cm² with an ITO on glass/Co(OH)₂/CoS electrode(0.09 cm²) according to a preferred embodiment of the invention. A Agwire was used as a reference electrode. The Co(OH)₂ layer waselectrodeposited on an ITO on glass electrode at a current density of 20mA/cm² during 30 seconds. The NaCl concentration in theelectrodepositing solution was 1 M. All the electrodes were immersed ina EMI-TFSI solution comprising 0.163 M of EMI-I and 10 mM of I₂. Thescanning speed was 100 mV/s.

As it can be seen from FIG. 18, a Pt electrode has been compared with anITO on glass/Co(OH)₂/CoS electrode in order to determine theelectrocatalytic properties of the latter. This figure shows that theoxidation and reduction potentials of the two electrodes are similar(see also Table 10). However, the CoS electrode demonstrates a highercurrent density than Pt, which is of a great interest. TABLE 10Electrode E_(pc1) (V) E_(pa1) (V) ΔE_(p1) (V) E_(pc2) (V) E_(pa2) (V)ΔE_(p2) (V) Pt 0.23 0.41 0.18 0.69 0.85 0.16 ITO/ 0.19 0.40 0.21 0.670.84 0.17 Co(OH)₂/ CoS

FIG. 19 shows still further cyclic voltammograms demonstrating theinfluence of the deposition time of the Co(OH)₂ layer on ITO on glass,to prepare ITO on glass/Co(OH)₂/CoS electrodes according to a preferredembodiment of the invention. The Co(OH)₂ layers were electrodeposited onan ITO on glass electrode at a current density of 20 mA/cm². The ITO onglass/Co(OH)₂/CoS electrodes have a surface area of 0.09 cm² forelectrodeposition time of 30 and 60 seconds, and 0.06 cm² for anelectrodeposition time of 90 seconds. A Ag wire was used as a referenceelectrode. All the electrodes were immersed in a EMI-TFSI solutioncomprising 0.163 M of EMI-I and 10 mM of I₂. The scanning speed was 100mV/s.

As it can be seen from FIG. 19, the ITO on glass/Co(OH)₂/CoS electrodes,which have been prepared by electrodepositing Co(OH)₂ for 60 or 90seconds, show higher current densities than the electrode prepared usingan electrodeposition time of 30 seconds. These results demonstrate thatthe roughness factor increases with the electrodeposition time.Regarding the oxidation and reduction potentials, a similarity isobserved for the three electrodes (Table 11). TABLE 11 Electrode E_(pc1)(V) E_(pa1) (V) ΔE_(p1) (V) E_(pc2) (V) E_(pa2) (V) ΔE_(p2) (V)ITO/Co(OH)₂/ 0.19 0.40 0.21 0.67 0.84 0.17 CoS 30 seconds ITO/Co(OH)₂/0.21 0.43 0.22 0.57 0.81 0.24 CoS 60 seconds ITO/Co(OH)₂/ 0.23 0.41 0.180.65 0.83 0.18 CoS 90 seconds

FIG. 20 and 21 represent cyclic voltammograms comparing an ITO onpolymer (polyethylene terephthalate) electrode having a surface area of0.05 cm² (FIG. 20) with an ITO on polymer (polyethyleneterephthalate)/Co(OH)₂/CoS electrode (0.05 cm²) (FIG. 21) according to apreferred embodiment of the invention. A silver wire was used as areference electrode. The Co(OH)₂ layer was electrodeposited on an ITO onpolymer electrode at a current density of 15 mA/cm² during 90 secondsusing a solution containing 1 M of NaCl. The reference and the testedelectrodes were immersed in a EMI-TFSI solution comprising 0.163 M ofEMI-I and 10 mM of I₂. The scanning speed was 100 mV/s.

As it can be seen from FIG. 20 and 21, an ITO on polymer electrode hasbeen compared with an ITO on polymer/Co(OH)₂/CoS electrode in order todetermine the electrocatalytic properties of the latter. The comparisonshows that the ITO on polymer/Co(OH)₂/CoS electrode acts as a very goodelectrocatalyst for the reduction of triiodide. In particular, thereduction of I₃ ⁻ is favored by 1.1 V and the oxidation of I⁻ is favoredby 0.65 V when using the CoS electrode instead of the ITO electrode. TheE_(pc1) and E_(pa1) of the CoS electrode are respectively 0.70 V and1.15 V vs Ag (Table 12). The ΔE_(p1) of the latter is thus 0.45 Vinstead of 2.20 V for ITO. TABLE 12 Electrode E_(pc1) (V) E_(pa1) (V)ΔE_(p1) (V) E_(pc2) (V) E_(pa2) (V) ΔE_(p2) (V) ITO on −0.40 1.80 2.20 —— — polymer ITO/ 0.70 1.15 0.45 1.50 1.75 0.25 Co(OH)₂/ CoS

While the invention has been described in connection with specificembodiments thereof, it will be understood that it is capable of furthermodifications and this application is intended to cover any variations,uses, or adaptations of the invention following, in general, theprinciples of the invention and including such departures from thepresent disclosure as come within known or customary practice within theart to which the invention pertains and as may be applied to theessential features hereinbefore set forth, and as follows in the scopeof the appended claims.

1. An electrode comprising: a non-conductive substrate; a first layerdisposed on said substrate, said layer comprising indium tin oxide orfluorine-doped SnO₂; and an second layer disposed on said first layer,said second layer comprising CoS, wherein said first and second layersare each substantially transparent.
 2. The electrode of claim 1, whereinsaid non-conductive substrate is a polymer substrate or a glasssubstrate.
 3. The electrode of claim 2, wherein said non-conductivesubstrate is a polymer substrate comprising a polymer is selected fromthe group consisting of polycarbonate, acetate, polyethyleneterephthalate, polyethylene naphthalate and polyimide.
 4. The electrodeof claim 1, wherein said second layer has a thickness of less than about5 μm.
 5. The electrode of claim 1, wherein said electrode has atransmittance of visible polychromatic light of at least 35%.
 6. Theelectrode of claim 1, wherein said electrode has a transmittance ofvisible polychromatic light of at least 60%.
 7. An indium tin oxideglass electrode or indium tin oxide polymer electrode having thereon asubstantially transparent layer comprising CoS.
 8. The electrode ofclaim 7, further including a Co(OH)₂ layer disposed between said indiumtin oxide glass electrode or said indium tin oxide polymer electrode,and said CoS layer.
 9. The electrode of claim 8, wherein said Co(OH)₂layer is substantially transparent.
 10. The electrode of claim 8,wherein said Co(OH)₂ layer has a thickness of less than about 5 μm. 11.The electrode of claim 8, wherein said Co(OH)₂ layer has a thickness ofabout 0.25 to about 4 μm.
 12. The electrode of claim 7, wherein saidelectrode has a transmittance of visible polychromatic light of at least35%.
 13. The electrode of claim 7, wherein said electrode has atransmittance of visible polychromatic light of at least 60%.
 14. Anelectrode comprising: a non-conductive substrate; a first layer disposedon said substrate, said first layer comprising indium tin oxide orfluorine-doped SnO₂; a second layer disposed on said first layer, saidsecond layer comprising Co(OH)₂; and a third layer disposed on saidsecond layer, said third layer comprising CoS, wherein said first,second and third layers are each substantially transparent.
 15. Theelectrode of claim 14, wherein said non-conductive substrate is apolymer or a glass substrate.
 16. The electrode of claim 15, whereinsaid non-conductive substrate is a polymer substrate comprising apolymer selected from the group consisting of polycarbonate, acetate,polyethylene terephthalate, polyethylene naphthalate and polyimide. 17.The electrode of claim 14, wherein said second layer has a thickness ofless than about 5 μm.
 18. The electrode of claim 14, wherein said secondlayer has a thickness of about 0.25 to about 4 μm.
 19. The electrode ofclaim 14, wherein said second layer has a thickness of about 0.5 toabout 2 μm.
 20. The electrode of claim 14, wherein said electrode has atransmittance of visible polychromatic light of at least 35%.
 21. Theelectrode of claim 14, wherein said electrode has a transmittance ofvisible polychromatic light of at least 45%.
 22. The electrode of claim14, wherein said electrode has a transmittance of visible polychromaticlight of at least 60%.
 23. A process for preparing an electrode,comprising the steps of: a) providing a non-conductive substrate, saidsubstrate having an indium tin oxide and/or a fluorine-doped SnO₂ layerthereon; b) electrodepositing a Co(OH)₂ layer on said indium tin oxideor fluorine-doped SnO₂ layer; and c) converting a portion of the Co(OH)₂layer into a layer of CoS.
 24. The process of claim 23, wherein step (b)is carried out by: i) using the substrate of step (a) as a cathode andproviding a cobalt electrode as an anode; ii) inserting said cathode andsaid anode into a cell having therein a solution comprising a cobaltsalt and a buffer; and iii) applying a galvanostatic current to thesolution thereby forming a layer of Co(OH)₂ on said cathode.
 25. Theprocess of claim 24, wherein said solution in step (ii) has a pH ofabout 6.0 to about 7.5.
 26. The process of claim 24, wherein the pH isabout 6.8 to about 7.5
 27. The process of claim 24, wherein said bufferis a NH₄Cl/NH₄OH buffer.
 28. The process of claim 24, wherein saidcobalt salt is selected from the group consisting of cobalt acetate,cobalt chloride, cobalt nitrate, cobalt sulphate and mixtures thereof.29. The process of claim 24, wherein said cobalt salt is cobaltsulphate.
 30. The process of claim 24, wherein said current in step(iii) has a density of about 15 to about 30 mA/cm².
 31. The process ofclaim 24, wherein said current in step (iii) has a density of about 10to about 15 mA/cm².
 32. The process of claim 24, wherein in step (iii)the current is applied for a period of time ranging from 1 to 120seconds.
 33. The process of claim 23, wherein in step (b) the Co(OH)₂layer electrodeposited on said substrate is substantially transparent.34. The process of claim 23, wherein step (c) is carried out bycontacting said layer of Co(OH)₂ with a basic solution comprising atleast one source of sulfur.
 35. The process of claim 34, wherein thebasic solution has a pH of at least 10.0.
 36. The process of claim 35,wherein the pH is about 13.0 to about 14.0.
 37. The process of claim 34,wherein said basic solution comprises S together with Li₂S, Na₂S, K₂S ormixtures thereof.
 38. The process of claim 37, wherein said basicsolution comprises S together with Na₂S.
 39. The process of claim 34,wherein said basic solution includes a base selected from the groupconsisting of LiOH, NaOH, NH₄OH, KOH and mixtures thereof.
 40. Theprocess of claim 34, wherein said non-conductive substrate is a polymersubstrate and said basic solution is a NH₄OH aqueous solution.
 41. Theprocess of claim 23, wherein said electrode has a transmittance ofvisible polychromatic light of at least 35%.
 42. A photovoltaic cellcomprising an electrode as defined in claim
 1. 43. A photovoltaic cellcomprising an electrode as defined in claim
 14. 44. A photovoltaic cellcomprising an anode, an electrolyte and, as a cathode, an electrode asdefined in claim
 1. 45. The photovoltaic cell of claim 44, wherein saidanode comprises n-CdSe.
 46. The photovoltaic cell of claim 44, whereinsaid electrolyte comprises a redox couple together with a solvent, apolymer, a gel or a combination thereof.
 47. The photovoltaic cell ofclaim 46, wherein said redox couple is R₁SM/(R₁S)₂ in which: M is ametal selected from the group consisting of Li, Na, K and Cs; R₁S⁻ is athiolate and (R₁S)₂ is a corresponding disulfide wherein R₁ is selectedfrom the group consisting of

wherein R₃ and R₄ are same or different and selected from the groupconsisting of a hydrogen atom, halogen atom, —NO₂, —OH, —CF₃—COR₆,—COOH, —COOR₆, —NHR₅, C₂-C₈ alkenyl, C₁-C₆ alkoxy, C₁-C₈ alkyl, C₂-C₈alkynyl, C₆-C₂₀ aralkyl, C₆-C₁₂ aryl, C₃-C₈ cycloalkyl and C₂-C₁₂heteroaryl comprising 1 to 4 heteroatoms selected from the groupconsisting of N, O and S, R₅ is a C₁-C₈ alkyl, C₆-C₁₂ aryl, C₃-C₈cycloalkyl, C₂-C₁₂ heteroaryl comprising 1 to 4 heteroatoms selectedfrom the group consisting of N, O and S, or any suitable protectinggroup for a nitrogen atom, R₆ is a C₁-C₈ alkyl, or a C₃-C₈ cycloalkyl,and X is N, O or S.
 48. The photovoltaic cell of claim 46, wherein saidredox couple is of formula M⁺I⁻/I₃ ⁻ or M⁺I⁻/I₂, wherein M⁺ is a metalselected from the group consisting of Li⁺, Na⁺, K⁺, Rb⁺ and Cs⁺.
 49. Thephotovoltaic cell of claim 46, wherein said redox couple is of formulaT⁺I⁻/I₃ ⁻ or T⁺I⁻/I₂, wherein T⁺ is selected from the group consistingof

wherein: R₁ and R₃ are same or different and selected from the groupconsisting of C₁-C₉ alkyl and benzyl; R₂ is a C₁-C₉ alkyl or H; R₄ andR₅ are same or different and represent a C₁-C₆ alkyl; R₆ and R₇ are sameor different and selected from the group consisting of a hydrogen atomand a C₁-C₇ alkyl; R₈ and R₉ are same or different and selected from thegroup consisting of hydrogen atom and C₁-C₄ alkyl; R₁₀, R₁₁ and R₁₂ aresame or different and represent a C₁-C₁₂ alkyl; R₁₃ is selected from thegroup consisting of a hydrogen atom, a halogen atom and a C₁-C₁₈ alkyl;R₁₄ and R₁₅ are same or different and represent a C₁-C₃ alkyl; and R₁₆,R₁₇, R₁₈ and R₁₉ are same or different and selected from the groupconsisting of hydrogen atom, C₁-C₁₂ alkyl, C₂-C₆ alkoxyalkyl, C₃ alkenyland C₃ alkynyl.
 50. A method for reducing disulfides into thiolatescomprising the step of electrochemically reducing said disulfides bymeans of an electrode as defined in claim
 1. 51. A method for oxidizingthiolates into disulfides comprising the step of electrochemicallyoxidizing said thiolates by means of an electrode as defined in claim 1.52. A method for catalyzing oxidation and reduction reactions of a redoxcouple of formula R₁SM/(R₁S)₂, comprising the step of submitting saidredox couple to an electrical current between at least two electrodeswherein at least one of said electrodes is an electrode as defined inclaim 1, and wherein M is a metal selected from the group consisting ofLi, Na, K and Cs; R₁S⁻ is a thiolate and (R₁S)₂ is a correspondingdisulfide wherein R₁ is selected from the group consisting of

wherein R₃ and R₄ are same or different and selected from the groupconsisting of a hydrogen atom, halogen atom, —NO₂, —OH, —CF₃—COR₆,—COOH, —COOR₆, —NHR₅, C₂-C₈ alkenyl, C₁-C₆ alkoxy, C₁-C₈ alkyl, C₂-C₈alkynyl, C₆-C₂₀ aralkyl, C₆-C₁₂ aryl, C₃-C₈ cycloalkyl and C₂-C₁₂heteroaryl comprising 1 to 4 heteroatoms selected from the groupconsisting of N, O and S, R₅ is a C₁-C₈ alkyl, C₆-C₁₂ aryl, C₃-C₈cycloalkyl, C₂-C₁₂ heteroaryl comprising 1 to 4 heteroatoms selectedfrom the group consisting of N, O and S, or any suitable protectinggroup for a nitrogen atom, R₆ is a C₁-C₈ alkyl, or a C₃-C₈ cycloalkyl,and X is N, O or S.
 53. A method for reducing a triiodide (I₃ ⁻) oriodine (I₂) into an iodide (I⁻) comprising the step of electrochemicallyreducing said triiodide or iodine by means of an electrode as defined inclaim
 1. 54. A method for oxidizing an iodide (I⁻) into a triiodide (I₃⁻) or iodine (I₂), comprising the step of electrochemically oxidizingsaid iodide by means of an electrode as defined in claim
 1. 55. A methodfor catalyzing oxidation and reduction reactions of a redox couple offormula M⁺I⁻/I₃ ⁻ or M⁺I⁻/I₂, comprising the step of submitting saidredox couple to an electrical current between at least two electrodeswherein at least one of said electrodes is an electrode as defined inclaim 1, and wherein M⁺ is a metal selected from the group consisting ofLi⁺, Na⁺, K⁺, Rb⁺ and Cs⁺.
 56. A method for catalyzing oxidation andreduction reactions of a redox couple of formula T⁺I⁻/I₃ ⁻ or T⁺I⁻/I₂;comprising the step of submitting said redox couple to an electricalcurrent between at least two electrodes wherein at least one of saidelectrodes is an electrode as defined in claim 1, and wherein T⁺ isselected from the group consisting of:

wherein R₁ and R₃ are same or different and selected from the groupconsisting of C₁-C₉ alkyl and benzyl; R₂ is a C₁-C₉ alkyl or H; R₄ andR₅ are same or different and represent a C₁-C₆ alkyl; R₆ and R₇ are sameor different and selected from the group consisting of a hydrogen atomand a C₁-C₇ alkyl; R₈ and R₉ are same or different and selected from thegroup consisting of hydrogen atom and C₁-C₄ alkyl; R₁₀, R₁₁, and R₁₂ aresame or different and represent a C₁-C₁₂ alkyl; R₁₃ is selected from thegroup consisting of a hydrogen atom, a halogen atom and a C₁-C₁₈ alkyl;R₁₄ and R₁₅ are same or different and represent a C₁-C₃ alkyl; and R₁₆,R₁₇, R₁₈ and R₁₉ are same or different and selected from the groupconsisting of hydrogen atom, C₁-C₁₂ alkyl, C₂-C₆ alkoxyalkyl, C₃ alkenyland C₃ alkynyl.